Iron-core toroidal current transformers (CTs) are widely used in the electric power industry to measure line current for protection and metering purposes. The line current is applied to a primary coil of the CT, and a reduced-magnitude version of the line current is produced on a secondary coil of the CT. This reduced-magnitude version of the line current is used as a measurement for protection and metering purposes.
One advantage of using an iron core CT is that most of the magnetic flux produced by a current in the primary winding passes through the secondary winding. Thus, iron-core CTs provide good flux linkage between the primary and secondary windings. Other advantages of using an iron-core toroidal CT include low production cost, inherent galvanic isolation, reliability, and ease of application.
However, a major disadvantage of iron-core toroidal CTs is that they are prone to current saturation. Such saturation occurs when currents exceeding a dynamic operating range of the CT cause magnetization of the core to be independent of the current, and thus produce distortion in the secondary signal. Saturation in these CTs is due mainly to two factors. First, the relationship between a magnetizing current (i.e., a current which produces the flux required to induce a voltage for transformer action) and a voltage applied to the secondary winding is non-linear. Second, iron-core toroidal CTs are able to retain a large magnetic flux density, or remanence, in their cores after removal of a current applied to the primary winding.